The invention relates to a fuel feed device for an aviation engine, more particularly for a gas turbine propulsion engine for airplanes.
In known fashion, a fuel feed device for a combustion chamber of a gas turbine propulsion engine for airplanes includes a pumping unit providing fuel under high pressure to a fuel metering unit or FMU, which delivers a regulated flow of fuel to a fuel feed line of the combustion chamber.
The pumping unit typically includes a high-pressure fuel pump which supplies the FMU and which receives fuel from a low-pressure fuel pump connected to a fuel reservoir. The high-pressure pump is customarily driven by coupling to a gear in a mechanical gearbox or accessory gearbox (AGB) connected by a power takeoff to a turbine shaft.
The FMU typically includes a variable-position metering valve controlled by an engine control computer or engine control unit (ECU) according to the fuel delivery desired in the feed line. A bypass valve is connected so as to maintain a constant pressure difference ΔP between the inlet and the outlet of the metering valve and to divert to the intake of the high pressure pump the excess fuel delivered thereby. A shutoff and atomization check valve is mounted on the feed line and is controlled by a servo-valve so as to open when the pressure in the feed line reaches a predetermined threshold and to close when said pressure falls below the opening threshold or in response to a shutoff command issued by the cockpit or the ECU.
In an aviation gas turbine engine, excessive rotation speed of a turbine shaft can have severe consequences, to wit causing the rotor disks mounted on the shaft to burst. Hence such an engine is customarily equipped with an overspeed protection device which receives information representing the rotation speed of an engine shaft and sends a fuel feed shutoff command when said rotation speed exceeds a predetermined threshold.
The overspeed safety function is provided by an electronic overspeed protection unit which can be built into the same case as that housing the ECU or be independent of the ECU, and which controls a fuel feed shutoff control device. The shutoff can be achieved by commanding the bypass valve of the FMU fully open so as to divert the entire flow provided by the high pressure pump and induce the closure of the shutoff and pressurization checkvalve by dropping the pressure in the fuel feed line.
It is also known in the art to build a fuel pumping unit with a jet pump inserted between the low-pressure fuel pump and the high-pressure fuel pump. The high-pressure fuel stream diverted by the bypass valve of the FMU is injected into a nozzle of the jet pump and drives the fuel delivered by the low-pressure fuel pump which is admitted through an intake in the jet pump surrounding the nozzle. The high-pressure fuel pump receives fuel at a pressure with a value between the low pressure and the high pressure, which results in less need for tapping mechanical power to drive the high-pressure fuel pump.
It has become apparent that such a use of a jet pump in association with an overspeed protection unit operating as described above raises a difficulty. As it happens, for the contribution from the jet pump to be effective, the nozzle of the primary intake of the jet pump must have a rather small flow area, which results in a considerable restriction in the bypass fuel return circuit. In the event of full opening of the bypass valve caused by detection of an overspeed, the presence of this restriction causes a large increase in the pressure in the return circuit which penalizes the flow capacity of the bypass valve and prevents complete diversion of the high-pressure fuel flow discharged by the high-pressure fuel pump. A considerable fraction of this high-pressure flow therefore passes through the metering valve and maintains sufficient pressure in the feed line that the closing of the shutoff and pressurization checkvalve cannot be ensured. The addition of an additional shutoff valve dedicated to overspeed protection and controlled by the overspeed protection unit can be considered, but at the price of increased bulk and cost.